Logical layers are merely a way of
organizing your code. Typical layers
include Presentation, Business and
Data – the same as the traditional
3-tier model. But when we’re talking
about layers, we’re only talking about
logical organization of code. In no
way is it implied that these layers
might run on different computers or in
different processes on a single
computer or even in a single process
on a single computer. All we are doing
is discussing a way of organizing a
code into a set of layers defined by
specific function.
Physical tiers however, are only about
where the code runs. Specifically,
tiers are places where layers are
deployed and where layers run. In
other words, tiers are the physical
deployment of layers.
Source: Rockford Lhotka, Should all apps be n-tier?
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